Stability constants of chromium(III) oxalate complexes in 1 m NaClO4 at 60°C

Stability constants of inert chromium(III) oxalate complexes in 1 m NaClO 4 medium at 60°C were determined by direct chemical analysis of the sum [C 2 O 4 2- ] + [HC 2 O 4 - ] + [H 2 C 2 O 4 ], i.e. the concentration of ligand not bound to the metal. Solutions containing different total molalities of metal, ligand and perchloric acid were equilibrated for several days at 60°C, then cooled to room temperature and analysed by precipitating the oxalate ion with Hg 2 2+ ion generated coulometrically. The end-point was assessed potentiometrically with the Hg 2 2+ /Hg half-cell. At room temperature the chromium(III)-oxalate complexes are sufficiently inert, only not bound to metal oxalate species react with Hg 2 2+ ion. The data, combined with the acidity constants of oxalic acid, are consistent with Cr 3+ + C 2 O 4 2- ⇄ CrC 2 O 4 + , log β 1 = 7.40±0.05 Cr 3+ + 2C 2 O 4 2- ⇄ Cr(C 2 O 4 ) 2 - , log β 2 = 13.54±0.05 Cr 3+ + 3C 2 O 4 2- ⇄ Cr(C 2 O 4 ) 3 3- , log β 3 = 18.07±0.05 Molar absorptivities of the various complex species, determined from the β n values and absorbency measurements (visible), match well literature data. The coincidence may be useful for validating the present model.